A typical electronic image sensor comprises a number of light sensitive picture elements (“pixels”) arranged in a two-dimensional array. Such an image sensor may be configured to produce a color image by forming an appropriate color filter array (CFA) over the pixels. Examples of image sensors of this type are disclosed in U.S. Patent Application Publication No. 2007/0024931, entitled “Image Sensor with Improved Light Sensitivity,” which is incorporated by reference herein.
As is well known, an image sensor may be implemented using complementary metal-oxide-semiconductor (CMOS) circuitry. In such an arrangement, each pixel typically comprises a photodiode and other circuitry elements that are formed in a silicon sensor layer on a silicon substrate. One or more dielectric layers are usually formed above the silicon sensor layer and may incorporate additional circuitry elements as well as multiple levels of metallization used to form interconnects. The side of the image sensor on which the dielectric layers and associated levels of metallization are formed is commonly referred to as the frontside, while the side having the silicon substrate is referred to as the backside.
In a frontside illuminated image sensor, light from a subject scene is incident on the frontside of the image sensor, and the silicon substrate is relatively thick. However, the presence of metallization level interconnects and various other features associated with the dielectric layers on the frontside of the image sensor can adversely impact the fill factor and quantum efficiency of the image sensor.
A backside illuminated image sensor addresses the fill factor and quantum efficiency issues associated with the frontside dielectric layers by thinning or removing the thick silicon substrate and arranging the image sensor such that light from a subject scene is incident on the backside of the image sensor. Thus, the incident light is no longer impacted by metallization level interconnects and other features of the dielectric layers, and fill factor and quantum efficiency are improved.
Backside illuminated image sensors can be difficult to process due to the thinning or removal of the silicon substrate. Conventional processing techniques, such as those disclosed in, for example, U.S. Patent Application Publication No. 2007/0194397, entitled “Photo-Sensor and Pixel Array with Backside Illumination and Method of Forming the Photo-Sensor,” can lead to increased die size or higher cost.
Another problem that arises in backside illuminated image sensors relates to so-called “dark” current, that is, current that is generated in the sensor even in the complete absence of incident light. Dark current adversely impacts sensor performance by making it more difficult to detect incident light. Such current can be particularly problematic in backside illuminated image sensors that are formed utilizing a silicon-on-insulator (SOI) image sensor wafer, as dark current tends to be generated at an interface between a sensor layer and a buried oxide layer. See, for example, T. Joy et al., “Development of a Production-Ready, Back-Illuminated CMOS Image Sensor with Small Pixels,” 2007 IEDM Technical Digest, pp. 1007-1009.
Accordingly, a need exists for processing techniques for forming backside illuminated image sensors with reduced dark current, particularly when using SOI image sensor wafers.